In both the 3-wheel and 4-wheel cases, the back 2 wheels are motor-driven. The front wheels are rigid castors wheels (they can rotate freely but not turn sideways)[...]Are you saying I should go for 3 wheels since less skid steering (friction)?

If each front wheel is mounted on a separate (good quality) castor, it won't make much difference with 3 or 4.

However... Did you consider the driven wheels at the approximate center, with a single castor at each end?That would keep the majority of the weight on, or close to, the driven wheels (thus it takes less power to turn) and the smaller (I assume) castor wheels won't dig into holes as easy, as it doesn't carry much load, just supporting the unit from tipping over.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I am afraid that swivel wheels would get stuck at cracks in the concrete. For e.g., one part of the ground could be slightly higher, and if the wheel 'bangs' flat against the higher part, then it might get stuck

I am afraid that swivel wheels would get stuck at cracks in the concrete. For e.g., one part of the ground could be slightly higher, and if the wheel 'bangs' flat against the higher part, then it might get stuck